This application claims the benefit of Japanese Application No. 2001-394409 filed Dec. 26, 2001.
The present invention relates to an X-ray CT (computerized tomography) apparatus for obtaining tomograms of the subject by X-ray irradiation.
An X-ray CT apparatus irradiates the subject (patient) with X-rays to detect differences in X-ray absorption rate among different tissues of the human body such as internal organs, blood and gray matter with an X-ray detector, and processes the detected differences with a computer to obtain (reconstruct) the images (X-ray tomograms) of the cut planes (planes in the respective sliced planes, i.e. sliced planes) of the photographed regions.
The X-ray CT apparatus has, available in the reconstruction of X-ray tomograms, a real-time display mode (Smart View) function in addition to a regular display mode for use in usual X-ray CT tomography, and one or the other of the two modes is selectively used according to the purpose.
The real-time display mode is for use in observing, for instance when the physician needles a patient who is the subject during axial scanning, the needled state, and allows, when the needle is moved, following the movement of the needle to update the display on a real-time basis.
The X-ray CT apparatus is provided with a segment data reconstruct processing (Segment Recon) capability as a data processing capability matching the real-time display mode. The segment data reconstruct processing takes place reconstruct X-ray tomograms on the basis of projection data obtained when a gantry provided with an X-ray tube and a detector revolves around the subject by (180+xcex1) degrees. The xe2x80x9cxcex1xe2x80x9d here is what is known as the Fan-Angle; while sufficient projection data for picking up parallel beam data for 180 degrees are needed to reconstruct X-ray tomograms, picking-up of an equivalent of xcex1 extra degrees is needed to pick up parallel beam data for 180 degrees.
Since the segment data reconstruct processing (Segment Recon) permits reconstruction of X-ray tomograms by picking up projection data for (180+xcex1) degrees, it is made possible to reduce the time required for picking up projection data at the time of start of scanning (i.e. to reduce the time required for reconstructing the first tomogram), and accordingly this can be considered a suitable processing method for the real-time display mode.
However, since the segment data reconstruct processing (Segment Recon) which is the processing method in the real-time display mode, use only a small quantity of projection data for reconstruction, it gives only poor image quality though it takes little time for reconstruction of the X-ray tomogram of the first frame, and moreover involves the problem of giving rise to xe2x80x9ctangent artifactsxe2x80x9d peculiar to segment data reconstruct processing (Segment Recon) and xe2x80x9cartifacts dependent on the tube-detector anglesxe2x80x9d. Tangent artifacts are noise arising linearly at the end of each part (e.g. a bone or each internal organ) in a reconstructed X-ray tomogram.
The artifacts dependent on the tube-detector angles are visual noise arising dependent on the rotational angles of the X-ray tube and the detector relative to the subject at the time of start of scanning in picking up projection data in the segment data reconstruct processing (Segment Recon). If the sectional shape of the subject is a true circle, no artifact dependent on the tube-detector angles will arise, but, if the sectional shape of the subject is an oval for instance, the extent to which artifacts arise will vary depending on whether the scanning is start in the direction of the minor axis or that of the major axis, and such artifacts can arise only when the segment data reconstruct processing (Segment Recon) is used.
It is therefore an object of the present invention to improve the image quality and to reduce artifacts in the real-time display mode of an X-ray CT apparatus while keeping short the time taken to reconstruct the first X-ray tomogram at the time of start of scanning.
In order to solve this problem, for instance an X-ray CT apparatus according to the invention has the following configuration. Thus, the X-ray CT apparatus has a rotary unit for causing an X-ray source and an X-ray detector for detecting X-rays from the X-ray source to rotate integrally around a subject, provided with: a full data reconstructing means for reconstructing tomograms on the basis of penetrating X-ray data of each view detected during a full turn around the subject, a segment data reconstructing means for reconstructing tomograms on the basis of penetrating X-ray data of each view detected during a less than full turn around the subject in a prescribed angle of rotation, and a change-over means for counting the tomograms reconstructed after the start of scanning, and changing over between the full data reconstructing means and the segment data reconstructing means according to the count.
According to the present invention, it is possible to improve image quality and reduce artifacts while keeping short the time taken to reconstruct the first X-ray tomogram at the time of start of scanning in the real-time display mode.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.